Alpine Hydrochemical Model (AHM)

The AHM (Alpine Hydrochemical Model) was developed and tested to asses the hydrologic and biogeochemical responses of seasonally snow covered alpine areas to changes in inputs of water, chemicals and energy. AHM is capable of incorporating a detailed understanding of watershed processes in order to simulate events critical to biota such as the ionic pulse associated with spring snowmelt, which is only a few days long and may involve only a portion of the catchment. The model computes integrated water and chemical balances for multiple terrestrial, stream and lake subunits within a watershed, each of which can have a unique and variable snow-covered area.
The model has thus far only been applied to The Emerald Lake Watershed in Sequoia National Park, CA USA. Wherever possible model parameters are based on field and lab measured properties of the basin. In its current application model capabilties include:

Tracking of chemical inputs from precipitation, dry deposition snowmelt, mineral weathering, flows external to the watershed, and user defined sources and sinks
Tracing surface and subsurface water and chemical movements through vegetation canopy, snowpack, soil litter, multiple soil layers, streamflow, and lake
Calculating chemical speciation, inculding: precipitates, exchange complexes, and acid-neutralizing capacity.
Simulating nitrogen reactions.
Using a snowmelt optimization procedure to aid in matching observed watershed outflows.
Modeling riparian areas.

Using one year of data for parameter estimation and a second for evaluation, the agreement between model and data was good.
A comparison of modeled and observed discharge and ANC (acid neutralizing capacity) for the Emerald Lake Watershed can be seen below.

Further application of AHM to forested catchments should be possible.

In actually running the model The user must first compile data about the watershed. Data requirements include:

Time series of snow covered area (SCA) data
Snow depth and snow water equivalence (SWE) data
Preciptitaion form, depth and chemical composition
Soil chemical and physical properties
Geologic makeup of rock and soil parent material
Lake and stream discharge and chemical data

Once this data has been collected it will be necessary to get the data into the proper format for AHM input. The existing files are good templates for new applications.

To download a copy of the AHM code and examples files: Ftp site for AHM The code is also available via anonymous ftp from virgo.hwr.arizona.edu. Log in as anonymous the AHM files are in pub/Alpine/ahm.
A user's manual is available from Roger Bales, Department of Hydrology and Water Resources, University of Arizona.
Wolford, RA. "Integrated Hydrochemical Modeling of an Alpine Watershed: Sierra Nevada, Califronia," Report 92-040, Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ, 1992.